Method for fabricating semiconductor thin film

ABSTRACT

An object of the present invention is to provide a technology of reducing a nickel element in the silicon film which is crystallized by using nickel. An extremely small amount of nickel is introduced into an amorphous silicon film which is formed on the glass substrate. Then this amorphous silicon film is crystallized by heating. At this time, the nickel element remains in the crystallized silicon film. Then an amorphous silicon film is formed on the surface of the silicon film crystallized with the action of nickel. Then the amorphous silicon film is further heat treated. By carrying out this heat treatment, the nickel element is dispersed from the crystallized silicon film into the amorphous silicon film with the result that the nickel density in the crystallized silicon film is lowered.

This application is a Division of application Ser. No. 08/536,977, filedSep. 29, 1995 now U.S. Pat No. 5,789,284.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for forming a siliconsemiconductor thin film which has a crystallinity and is formed on asubstrate having an insulation surface of a glass substrate or the like.

2. Description of the Related Art

In recent years, attention is paid on a technology for constructing athin film transistor by using a silicon thin film which is formed on aglass substrate. This thin film transistor is primarily used in anactive matrix type liquid crystal electro-optical device and other thinfilm integrated circuits. The liquid crystal electro-optical devicechanges optical characteristics of a liquid crystal thereby displayingan image by charging a liquid crystal into a pair of glass substratesand applying an electric field.

In particular, the active matrix type liquid crystal display deviceusing a thin film transistor is characterized in that the thin filmtransistor is arranged in each pixel, an electric charge held in a pixelelectrode is controlled by using the thin film transistor as a switch.Since the active matrix type liquid crystal display device is capable ofdisplaying a fine image at a high speed, the device can be used indisplays for various electronic apparatuses (for example, a portableword processor, and a portable computer or the like).

As a thin film transistor used in the active matrix type liquid crystaldisplay device, an amorphous silicon thin film is commonly used.However, the thin film transistor using the amorphous silicon thin filmhas the following problems.

(1) The liquid crystal thin film transistor has low characteristics andcannot display a higher quality image.

(2) The liquid crystal thin film transistor cannot constitute aperipheral circuit for driving a thin film transistor arranged on apixel.

The aforementioned second problem can be considered by dividing theproblem into the following two aspects. One aspect of the problem isthat since a P-channel type thin film transistor cannot be used forpractical purposes with the thin film transistor using an amorphoussilicon thin film, a CMOS circuit cannot be constituted. Another aspectof the problem is that since the thin film transistor using an amorphoussilicon thin film cannot be operated at a high speed, and a largecurrent cannot flow in the thin film transistor, a peripheral drivingcircuit cannot be assembled.

Means for solving the aforementioned problems include a technology forforming a thin film transistor by using a crystalline silicon thin film.The methods for obtaining a crystalline thin film include a method forheat treating an amorphous silicon film and a method for irradiating theamorphous silicon thin film with laser light.

SUMMARY OF THE INVENTION

However, a method for crystallizing am amorphous silicon film by heattreatment in the prior art has the following problem.

In the case where a thin film transistor is constituted which is used ina liquid crystal electro-optical apparatus, it is demanded that the thinfilm transistor is formed on a translucent substrate. Examples of thetranslucent substrate include a quartz substrate and a glass substrate.However, the quartz substrate is expensive and cannot be used in theliquid crystal electro-optical device which has a large technologicalproblem of a cost reduction. Consequently, the glass substrate iscommonly used. However, it has a problem of a low heat resistance.

Generally, as the glass substrate used in the liquid crystalelectro-optical device, Corning 7059 glass substrate is used. The strainpoint of this glass substrate is 593° C. When the substrate is heattreated at this temperature or more, the shrinkage or the deformation ofthe substrate becomes conspicuous. In recent years, the liquid crystalelectro-optical device tends to have a larger area and the shrinkage andthe deformation of the substrate must be suppressed as much as possible.

However, it has been proved in the experiment that a temperature of 600°C. or more is required to crystallize the amorphous silicon film byheating. It is also made clear that tens of hours are required forheating. A large area glass substrate cannot be subjected to such hightemperature and long hour heating at all.

Further, a technology of crystallizing the amorphous silicon film bylaser light irradiation is also known. However, it is difficult as apractical problem to irradiate uniformly a large area with laser lightand to irradiate the area while keeping a definite level of irradiationpower.

An object of the present invention is to solve the aforementionedproblems and to provide a technology of transforming the amorphoussilicon film into a crystalline silicon film by heat treatment at anextremely low temperature.

In particular, an object of the invention is to provide a crystallinesilicon thin film which is capable of constituting a thin filmtransistor with a high performance characteristics.

In accordance with a major aspect of the present invention, there isprovided a method for fabricating a semiconductor thin film comprisingthe steps of:

introducing into an amorphous silicon film a metal element whichpromotes the crystallization of silicon;

obtaining a crystalline silicon film by crystallizing the aforementionedamorphous silicon film by heat treatment;

forming a metal element diffusion film on said crystalline silicon film;

diffusing the aforementioned metal element into the aforementioned metalelement diffusion film; and

removing the metal element diffusion film into which the metal elementhas been diffused.

In the aforementioned structure, examples of the amorphous silicon filminclude a film which is formed by the plasma CVD or by the low pressurethermal CVD on a glass substrate or on a glass substrate on which aninsulating film is formed.

Further, examples of the metal element which promotes thecrystallization of the aforementioned silicon include one or more kindsof elements selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.Among these metal elements, the most effective metal element is nickel(Ni).

Methods for introducing the metal element which promotes thecrystallization of the aforementioned silicon include a method forproviding these metal layers or a layer including the metal on thesurface of the amorphous silicon film. Specifically, the methods includea method for forming a metal element layer or a layer including a metalelement by the CVD, the sputtering process, vapor deposition or thelike, and a method for coating a solution containing a metal element onthe amorphous silicon film. In particular, since the latter method usingthe latter solution enables easily controlling a density of the metalelement, the latter method is more favorable than the former method. Inaddition, since the metal element can be held uniformly in contact withthe surface of the amorphous silicon film in the latter method, thelatter method is very favorable in this respect, too. For reference,when the aforementioned CVD and the sputtering process, vapor depositionor the like is used, it is difficult to form an extremely thin uniformfilm. Consequently, there is a problem in that the metal element isnon-uniformly present on the amorphous silicon film, and the metalelement is liable to be deviated at the time of the crystal growth.

To crystallize by heating the silicon film to which the metal elementpromoting the crystallization of silicon is introduced, the silicon filmmay be heated at a temperature of 450° C. or more. The upper limit ofthis heating temperature is limited by the heat-resistant temperature ofthe glass substrate used as the substrate. In the case of the glasssubstrate, the heat resistant temperature can be regarded as the strainpoint of glass. When materials such as quartz substrate or the likewhich can endure a temperature of 1000° C. or more, the heatingtemperature in heating can be heightened in accordance with the heatinsulating temperature.

As one example of heat treatment, it is appropriate to set thetemperature to about 550° C. from the viewpoint of the heat resistanceand the productivity of the glass substrate.

An amorphous silicon film used as the metal element diffusion film andformed on crystalline silicon film crystallized as a result of heattreatment may be formed by the general CVD process. For example, thesame method as used for forming the crystalline silicon film byintroducing the metal element into the amorphous silicon starting filmand crystallizing the amorphous silicon starting film by heating isused.

However, more preferably, the film may be of the quality such that adefect density is high and the metal element can be easily trapped. Thisis because the metal element in the crystalline silicon film can beeasily dispersed in the metal element diffusion film comprising silicon.

The defect density can be set to a high level by using such means asforming the film only of silane without using. hydrogen in accordancewith the plasma CVD, using the sputtering method, or lowering thetemperature at which the film is formed in accordance with the plasmaCVD.

It is more advantageous to increase the thickness of this amorphoussilicon film with respect to the thickness of the crystalline siliconfilm. This is because when the amorphous silicon film is thicker thanthe crystalline silicon film, the volume ratio with respect to thecrystalline silicon film can be increased, and more metal elements canbe dispersed in the amorphous silicon film.

A polycrystalline silicon film and an amorphous Si_(x)Gi_(1-x) film(0<x<1) can be used as the metal element diffusion film. Thepolycrystalline silicon film can be formed by low pressure CVD. Theamorphous Si_(x)Ge_(1-x) film can be formed by plasma CVD using silane(SiH₄) and germane (GeH₄) as a raw material gas.

The step of diffusing (absorbing) the metal element in the crystallinesilicon film is carried out by heat treatment. Since the metal elementin the crystalline silicon film is diffused into the metal elementdiffusion film by the heat treatment, the metal element concentration islowered in the crystalline silicon film.

Next, the metal element diffusion film into which the metal element hasbeen diffused is removed. This removal can be a selective etching of themetal element diffusion film by forming an oxide film as an etchingstopper on the silicon film to be crystallized.

Concrete constitution of this method is described using FIG. 1. First, acrystalline silicon film 105 is formed on a glass substrate 101 usingnickel as a metal element which promotes crystallization of silicon.Reference numeral 102 designates a base silicon oxide film. Heattreatment is used for the crystallization. (FIG. 1(B))

Next as shown in FIG. 1(C), an oxide film 106 is formed, and anamorphous silicon film 107 is formed as the metal element diffusion filmand heat treated.

This heat treatment can be classified into two methods; one method isthe one which is carried out at a temperature (generally, 450° C. orless) at which the amorphous silicon film is not crystallized while theother method is one which is carried out at a temperature (generally,450° C. or more, and preferably 500° C. or more) at which the amorphoussilicon film is crystallized.

When the heat treatment is carried out at a temperature at which theamorphous silicon film 107 provided on the crystalline silicon film 105is not crystallized, the temperature of the heat treatment is 400° C. to450° C. and the heating duration is 5 minutes to 10 hours. The metalelement in the amorphous silicon film is gradually absorbed into theamorphous silicon film 107 by this heat treatment. Consequently, whenthe heat treatment is carried out over a long period, the density of themetal element in the crystalline silicon film 105 can be graduallydecreased.

By removing the amorphous silicon film 107 using the oxide film 106 asan etching stopper, a crystalline silicon film 108 containing a metalelement at a low concentration therein as compared with the metalelement concentration in the amorphous silicon film 107 can be obtained.(FIG. 1(D)) This is because the silicon atom is present in the amorphoussilicon film 107 in the state that the silicon atom is liable to beconnected with the metal element (in the amorphous state, a largequantity of unpaired bonds are present). Further, this action can beconspicuously obtained when the defect density is artificially increasedin the amorphous silicon film 107.

In the meantime, when the amorphous silicon film is heated at atemperature at which the crystallization of the amorphous silicon film107 provided on the crystalline silicon film 105 proceeds, thedispersion of the metal element is ostensibly suspended in the state inwhich the amorphous silicon 107 is crystallized. Then when the averagevalue of the density of the metal element in the crystalline siliconfilm 105 and the density of the metal element in the silicon film 107for absorbing the metal element (crystallized in heat treatment) becomeapproximately equal to each other, the dispersion of the metal elementis ostensibly suspended.

However, it has been made clear that the metal element is locallyconcentrated in the crystalline silicon film 105. This method becomeseffective for suppressing this phenomenon. Therefore, this is a methodwhich is intended to produce a state in which no concentration of themetal element exists in the silicon film to be used in the fabricationof the device by using the phenomenon that the metal element isconcentrated on the tip part of the crystal growth to dispel the tip ofthe crystal growth to the silicon film to be removed later.

Then, heat treatment is carried out at the temperature at which theamorphous silicon film 107 is crystallized to crystallize the amorphoussilicon film 107. At this occasion, the crystal growth proceeds from asurface at which the silicon film 107 contacts the-oxide film 106 to theexposed surface thereof. Then, at the same time with this crystalgrowth, a portion where the metal element is concentrated moves in thesilicon film 107. As a consequence, a portion where a nickel element isconcentrated is dispelled from the silicon film 105 so that the nickelelement exists in the silicon film 107 (particularly on the surfacethereof). Then, a crystalline silicon film 108 free from an area wherethe nickel element is deviated can be obtained by removing the siliconfilm 107 using the oxide film 106 as an etching stopper (FIG. 1(D)).

An amorphous silicon film is formed on the surface of the crystallinesilicon film crystallized with the action of metal which promotes thecrystallization, followed by heat treatment to disperse the metalelement into the amorphous silicon film. In this manner, the metalelement in the crystalline silicon film can be virtually sucked outthereby making it possible to obtain a crystalline silicon film having alow density of metal element and having a crystallinity.

In addition, since all these steps can be carried out at a temperaturesuch as 550° C. or less which the glass substrate can endure, the stepsare extremely useful in the formation of a crystalline thin film siliconsemiconductor used for constructing a thin film transistor used in aliquid crystal electro-optical device using, for example, a glasssubstrate.

To facilitate the removal of the silicon film for the absorption of themetal element, it is effective to form an oxide film on the crystallinesilicon film. Since the oxide film has a selectivity with respect to anetchant (for example, hydradine and ClF₃), the oxide film can serve asan etching stopper.

Further, on a surface of the crystalline silicon film (first siliconfilm) crystallized with the action of the metal promoting thecrystallization, an amorphous silicon film (second silicon film) isformed. Thereafter, heat treatment is carried out to crystallize thesecond layer amorphous silicon film with the result that theconcentrated portion of the metal element which is present in the firstsilicon film can be dispelled into the second silicon film therebysuppressing the deviation of the metal element in the first siliconfilm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(A) to 1(D) are views showing a step of fabricating a crystallinesilicon film.

FIG. 2 is a view showing a density distribution of a nickel element.

FIG. 3 is a view showing a density distribution of the nickel element.

FIG. 4 is a view showing a density distribution of the nickel element.

FIGS. 5(A) to 5(D) are views showing a step of fabricating a thin filmtransistor.

FIGS. 6(A) to 6(D) are views showing a step of fabricating a crystallinesilicon film.

FIGS. 7(A) to 7(C) are views showing a step of fabricating a thin filmtransistor.

FIGS. 8(A) to 8(C) are views showing a step of fabricating a crystallinesilicon film.

FIGS. 9(A) to 9(C) are views showing a step of fabricating a crystallinesilicon film.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiment 1

Embodiment 1 relates to a technology of dispersing a nickel element intoan amorphous silicon film from a crystalline silicon film (allowing theamorphous silicon film to such the nickel element) to lower a density ofthe nickel element in the crystalline silicon film as a consequence, byforming an amorphous silicon film, introducing into the amorphoussilicon film a metal film which promotes the crystallization of siliconfilm, forming an amorphous silicon film on a crystallized silicon film(crystalline silicon film) via an oxide film (silicon oxide film),followed by performing heat treatment again.

FIG. 1 shows a step of fabricating a crystalline silicon film describedin this embodiment. In the beginning, on Corning 7059 glass substrate101 (having a strain point of 593° C.), a silicon oxide film 102 isformed to a thickness of 3000 Å as a base film. The silicon oxide film102 is intended to prevent impurities and alkaline ions from beingdispersed into the semiconductor thin film from the glass substrate 101.Then, the amorphous silicon film 103 is formed to a thickness of 600 Åby the plasma CVD and the low pressure thermal CVD. Then, a nickelacetate solution which is adjusted to a predetermined nickel density isdripped on the amorphous silicon film 103 to form a water film 104.Then, the spinner 100 is used for spin coating to produce a state inwhich the nickel element is held in contact with the surface of theamorphous silicon film 103 (FIG. 1(A)).

Subsequently, the amorphous silicon film 103 is crystallized by heattreatment to obtain a crystalline silicon film 105. The heat treatmentcan be carried out at a heating temperature of 450° C. or more, orpreferably, 500° C. or more. However, in consideration of heatresistance of the glass substrate 101, preferably, the temperature isset to a strain point of the glass substrate 101 or less. Incidentally,when the heat treatment is carried out at a temperature of 500° C. orless, the heat treatment requires tens of hours or more. Thus, themethod is not practical.

The density of nickel in this crystalline silicon film 105 is requiredto be set to 1×10₁₆ atom cm⁻³ to 5×10¹⁹ atom cm⁻³ if possible.Consequently, it is necessary to adjust the nickel density in thecrystalline silicon film 105 thus obtained to the aforementioned rangeat the step shown in FIG. 1(A). Incidentally, the nickel density isdefined as a minimum value measured by using the SIMS (secondary ionmass spectrometer).

When the crystalline silicon film 105 is obtained, a silicon oxide film106 is formed on the surface of the film 105. The thickness of thesilicon oxide film 106 may be set to about tens of Å to about 100 Å.Such a thin film is formed because the nickel element in the crystallinesilicon film is required to move via the silicon oxide film 106. Here,an extremely thin silicon oxide film 106 is formed by the UV lightirradiation in the air. Even when this thin silicon oxide film 106 is asthick as a natural oxide film, it has been made clear that the thin film106 serves as an etching stopper in the subsequent step of etching ofthe amorphous silicon film (denoted by 107). Thus the thin film 106 maybe as thick as the aforementioned thickness. Here, the silicon oxidefilm 106 is formed by using the UV oxidation process, but the siliconoxide film may be formed by the thermal oxidation process.

This silicon oxide film 106 serves as an etching stopper in the lateretching step, and the silicon oxide film 106 can be used as long as itprovides a selectivity with respect to the crystalline silicon film 105.For example, in the place of the silicon oxide film 106, an extremelythin silicon nitride film can be used.

Subsequently, the amorphous silicon film 107 is formed to a thickness of600 Å by the plasma CVD and low pressure CVD.

FIG. 2 shows a density distribution of the nickel element in thedirection of the film thickness by using SIMS (secondary ion massspectrometer). What is shown in FIG. 2 is a distribution of the nickelelement in the depth direction from the surface of the amorphous siliconfilm 107. As apparent in FIG. 2, it is made clear that the nickelelement in the amorphous silicon film 107 is less than the measurementlimit (here, in this case, 1×10₁₇ atom cm⁻³), and a maximum of about5×10₁₈ atom cm⁻³ of the nickel element is present in the crystallinesilicon film 105.

Then the nickel element in the crystalline silicon film 105 is dispersedin the amorphous silicon film 107 via an oxide film 106 by heattreatment. This step can be understood as a step of sucking out thenickel element in the crystalline silicon film 105 (FIG. 1(C)).

This step of heat treatment is carried out at a temperature of from 400to 450° C. at which the amorphous silicon film 107 is not crystallized.In this embodiment, the heat treatment is carried out at 450° C. for twohours. When this heat treatment is carried out, the nickel element inthe crystalline silicon film 105 is dispersed in the amorphous siliconfilm 107 thereby making it possible to lower the density of the nickelelement in the crystalline silicon film 105.

Generally, when the thickness of the amorphous silicon film 107 is setto not less than the thickness of the crystalline silicon film 105, thedensity of the nickel element in the crystalline silicon film 105 can beset to a half or less by carrying out the aforementioned heat treatment.

FIG. 3 shows a density distribution of the nickel element in the filmthickness direction in a state in which the heat treatment is carriedout for two hours. Data shown in FIG. 3 is made with the samemeasurement method as data shown in FIG. 2.

As is apparent in FIG. 3, nickel is dispersed in the amorphous siliconfilm 107. However, it can be seen in FIG. 3 that the density of nickelis somewhat higher in the crystalline silicon film 105. From FIG. 3, itcan be understood that the nickel element in the crystalline film 105 isabsorbed in the amorphous silicon film 107 at the step of heat treatmentshown in FIG. 1(C).

FIG. 4 shows a density distribution of nickel in four hour heattreatment at 450° C. when two hour heat treatment is further carried outat 450° C. after data shown in FIG. 3 is obtained. (Finally, four hourtreatment is carried out at 450° C.) As is apparent in a comparisonbetween FIGS. 3 and 4, it can be seen that the nickel element in thecrystalline silicon film 105 is gradually sucked into the amorphoussilicon film 107. It is considered that a large amount of unpaired bondsand a large number of silicon atoms to which nickel can be easily bondedexist in the amorphous silicon film 107. Further, the nickel density inthe crystalline silicon film 105 can be gradually lowered by carryingout a long hour heat treatment. Such an action is a feature that cannotbe observed when the amorphous silicon film 107 is crystallized.

Then, the amorphous silicon film 107 is removed by etching. Here, as anetchant of the amorphous silicon film 107, hydradine (N₂H₆) is used.When hydradine is used as an etchant, the amorphous silicon film 107 hasa faster etching rate than the crystalline silicon film 105. Further, inthe present embodiment, an oxide silicon film 106 which cannot be etchedwith hydradine (the etching rate is extremely low, and in a relativeviewpoint, it can be regarded that etching cannot be performed) isformed on the crystalline silicon film as an etching stopper. Therefore,only the amorphous silicon film 107 which has sucked out nickel can beselectively removed. Incidentally, dry etching may be used for etchingthe amorphous silicon film 107.

Subsequently, the silicon oxide film 106 is removed with buffer acid andfluorine nitride to obtain a crystalline silicon film 108 in which thedensity of the contained nickel element as shown in FIG. 1(D) can belowered. The density of the nickel element in this crystalline siliconfilm 108 is, for example, about 3×10¹⁸ atom cm⁻³. As apparent in thecomparison with FIG. 2, this value means that the density of the nickelelement can be lowered to a half (a half or less in average).

In this embodiment, the thickness of the amorphous silicon film 107which is formed on the crystalline silicon film 105 is set to the samethickness as the thickness of the crystalline silicon film 105. However,the density of the nickel element which is contained in the crystallinesilicon film 108 which is finally obtained can be further lowered byincreasing the thickness of the amorphous silicon film 107. That is, alarger amount of the nickel element can be sucked out into the amorphoussilicon film 107 by increasing the volume of the amorphous silicon film107 compared with the volume of the crystalline silicon film 105.

With the adoption of the structure of this embodiment, the density ofnickel in the crystalline silicon film 108 thus obtained can be set to5×10_(18 a)tom cm⁻³ or less.

Embodiment 2

Embodiment 2 is characterized in that the step of heat treatment in thefabrication process in embodiment 1 is carried out by setting theconditions as follows; heating temperature is 550° C., and heatingduration is four hours. When the heat treatment step shown in FIG. 1(C)is carried out at 550° C. for four hours, the amorphous silicon filmdenoted by 107 is crystallized with the action of the nickel elementwhich is dispersed from the crystalline silicon film 105.

At this time, the crystalline advances in a direction of heading fromthe crystalline silicon film 105 to the amorphous silicon film 107 viathe oxide film 106. As described above, the metal element which promotesthe crystallization of silicon tends to be concentrated at the tipportion of the crystal growth. Consequently, an area where the nickelelement is concentrated exist in the crystallized silicon film 107(which is transformed into a crystalline film at this stage). Thennaturally the density of nickel in the crystalline silicon film isdecreased.

Further, the nickel element deviated area which is present on thesurface of the crystalline silicon film 105 moves together with the tipportion of the crystal growth along with the advancement of the progressof the crystallization of the amorphous silicon film 107. That is, thenickel element deviated area will be present in the silicon film 107(which is here designated as a crystallized state) after the terminationof the crystallization. Consequently, the nickel element deviated areaon the surface of the crystalline silicon film 105 can be eliminated.

In the case where the amorphous silicon film 107 has been crystallizedin this manner, it is feared whether or not this crystallized siliconfilm can be selectively removed. Since the silicon oxide film 106 isformed which serves as an etching stopper, only the silicon film denotedby 107 (crystallized in this case crystallized) can be selectivelyremoved. In other words, when etching is carried out using hydradine andClF₃ gas, the etching rate of oxide film denoted by reference numeral106 is extremely small compared with the etching rate of the siliconfilm denoted by reference numeral 107. Thus the etching is suspendedwhen the etching of the silicon film 107 is ended.

When the structure shown in embodiment 2 is adopted, the heat treatmentstep shown in FIG. 1(C) can be performed under the same condition as theheat treatment as FIG. 1(B).

Embodiment 3

Embodiment 3 shows an example of fabricating a thin film transistor byusing the crystalline silicon film which is obtained by the method forfabricating the silicon film shown in embodiments 1 and 2. Referring toFIG, 5, there is shown a method for fabricating the thin filmtransistor. In the beginning, a crystalline silicon film 503 is formedon the glass substrate 501 on which a base film 502 is formed by using amethod shown in embodiments 1 and 2. (FIG. 5(A)) Subsequently, thecrystalline silicon film 503 thus obtained is patterned to form anactive layer of the thin film transistor as shown by reference numeral504. Then a silicon oxide film 505 is formed to a thickness of 1000 Åwhich functions as a gate insulating film by the plasma CVD and the lowpressure thermal CVD. (FIG. 5(B))

Then an aluminum film containing scandium is formed to a thickness of6000 Å followed by patterning the film to form a gate electrode denotedby reference numeral 506. Further, in the electrolyte, anodic oxidationis carried out by using the gate electrode 506 as an anode to form anoxide layer 507. The thickness of the oxide layer 507 is set to 2000 Å.It is possible to form an offset gate area to a thickness of this oxidelayer 507 at the subsequent step (FIG. 5(C)).

Then, impurity ions are doped into the active layer 504. Here,phosphorus ions are doped as an impurity ion. At this step, phosphorusions are doped into an area denoted by reference numerals 508 and 511.The area denoted by reference numerals 508 and 511 constitute a sourcearea and a drain area. Further, an area denoted by reference numeral 509constitutes an offset area. Further, the area denoted by referencenumeral 510 constitutes a channel-formation area.

After the completion of the impurity ion doping, the silicon film isirradiated with laser light to activate the doped ions and to anneal thesource and drain areas 508 and 511 that has been damaged at the time ofion doping. (FIG. 5(C)) Subsequently, a silicon oxide film 512 is formedas an interlayer insulating film and then a contact hole is formed toform a source electrode 513 and a drain electrode 514 by using aluminum.Further, a thin film transistor is completed by heat treatment in thehydrogen atmosphere of 350° C. (FIG. 5(D)).

Embodiment 4

Embodiment 4 relates to a technology for obtaining a crystalline siliconfilm which has crystal grown in a direction parallel to the substrateand then lowering the density of nickel in this crystalline silicon filmby selectively introducing nickel which is a metal element that promotesthe crystallization of silicon.

In the beginning, a silicon oxide film is formed to have a thickness of3000 Å as a base film on the glass substrate 601 by the sputteringmethod. Subsequently, the amorphous silicon film 603 is formed to have athickness of 500 Å by the plasma CVD or the low pressure thermal CVD.

Subsequently, the amorphous silicon film is irradiated with UV light inthe atmosphere of oxygen to form an extremely thin oxide film (notshown) on the surface of the amorphous silicon film 603. This oxide filmis intended to improve the moisture characteristics of the solution atthe subsequent step of coating the solution.

Then, a resist is used to form a mask 604. The area 605 exposed by theresist mask 604 has a slit-like configuration with a longitudinaldirection from the surface side of the paper shown in FIG. 6(A) to therear surface of FIG. 6(A).

Then, a nickel acetate solution containing a predetermined density ofnickel is dripped to form a water film 606.

Further, a spinner 600 is used for spin coating to produce a state inwhich the nickel element is held in contact with the surface of theamorphous silicon film 603 via an oxide film (not shown) in an areadenoted by reference numeral 605.

Then, the resist mask 604 is removed. Then the amorphous silicon film603 is heat treated to be crystallized. Here, in the area designated byreference numeral 605, the nickel element is dispersed from a state inwhich the nickel element is held in contact with the amorphous siliconfilm 603 via the oxide film (not shown) into the amorphous silicon filmvia the oxide film (not shown). Then, as denoted by an arrow 607, thecrystal growth proceeds in a direction parallel to the substrate. Thiscrystal growth advances in a column-like or a needle-like configuration.In embodiment 4, since the area denoted by reference numeral 605 has aslit-like configuration with a longitudinal direction from the surfaceof the paper shown in FIG. 6(A) to the rear surface of FIG. 6(A). Thecrystal growth as shown by the arrow 607 advances approximately in onedirection. This crystal growth can be carried out over tens of μm to 100μm or more. (FIG. 6(B))

In this manner, the crystalline silicon film 608 is obtained as shown inFIG. 6(C). Then, the oxide film 609 is formed to have a thickness of 50Å by the thermal oxidization method. Moreover, an amorphous silicon film610 having a thickness of 1000 Å is formed by the plasma CVD or the lowpressure thermal CVD.

Then, the heat treatment is carried out at 450° C. for two hours, andcauses the nickel element in the crystalline silicon film 608 to bedispersed in the amorphous silicon film 610 via an oxide film 609. Theamorphous silicon film 610 is etched with ClF₃ gas, and then the oxidefilm 609 is removed with buffer fluoric acid. Thus the crystallinesilicon film 611 having a lowered nickel density as shown in FIG. 6(D)can be obtained. This crystalline silicon film 611 is characterized inthat the crystalline silicon film 611 has an area which is crystal grownin a direction parallel to the substrate as denoted by reference numeral607 and, further, the nickel density is low.

An experiment has made clear that somewhat large amount of nickel whichis introduced into the area shown by reference numeral 605 can have alonger distance of crystal growth in a direction parallel to thesubstrate (which is referred to a longitudinal direction growth).However, in the meantime, a larger amount of the introduced nickelelement can be a factor of heightening the nickel element concentrationin the silicon film. Therefore the large amount of introduced nickel isnot favorable. This is because when the nickel density in the film isincreased (5×10¹⁹ atom cm⁻³ or more in the experiment), a problembecomes apparent in that the characteristic of the silicon film isdeteriorated or the operation of thin film transistors becomes unstableand the deterioration of characteristic becomes drastic.

However, as shown in Embodiment 4, the present invention can meet thefollowing two demands at the same time by removing the nickel elementafter the completion of crystallization; a demand that the distance ofcrystal growth in the longitudinal direction be prolonged in thehorizontal direction, and a demand that the nickel density (density ofmetal element) in the obtained crystalline silicon film 611 be loweredas much as possible.

Embodiment 5

Embodiment 5 shows an example in which a crystalline silicon filmobtained in embodiment 4 is used to constitute a thin film transistor.FIG. 7 shows a fabrication step in embodiment 5. In the beginning, acrystalline silicon film as shown by reference numeral 611 in FIG. 6(D)has an area which has crystal grown in a direction parallel to thesubstrate.

Subsequently, as shown in FIG. 7(A), an active layer 703 of the thinfilm transistor is formed by patterning the crystalline silicon film. InFIG. 7(A), reference numeral 701 denotes a glass substrate whilereference numeral 702 denotes a silicon oxide film as the base film.

Here, it is important that the beginning and the end of the crystalgrowth in the crystal growth shown in FIG. 6(B) do not exist in theactive layer 703. This is because a high density of nickel is containedin the beginning and the end of the crystal growth.

Further, a silicon oxide film 704 is formed which functions as a gateinsulating film to a thickness of 1000 Å by the plasma CVD. FIG. 7(A))

Subsequently, a film which primarily comprises aluminum is formedfollowed by patterning the film to form a gate electrode 705. Then, anoxide layer 706 is formed by anodic oxidation in the electrolyte usingthe gate electrode 705 as an anode. An offset gate area can be formed bythe impurity ion doping to a thickness of the oxide layer 706 at thesubsequent step of impurity ion doping. (FIG. 7(B)).

Then, as an impurity ion, phosphorus ions are doped. At this step, asource area 707 and a drain area 710 are formed. Further, an offset gatearea 708 and a channel formation area 709 are formed. After the impurityion doping is completed, the source and the drain areas 707 and 710 areformed by irradiating a laser light or an intense light to activate thesource and the drain areas 707 and 710.

Then, the silicon oxide film 711 which constitutes an interlayerinsulating film is formed to a thickness of 6000 Å by plasma CVD. Then,after a contact hole is formed, the source electrode 712 and the drainelectrode 713 are formed. Thus, a thin film transistor is completed.

Embodiment 6

Embodiment 6 is characterized in that heat treatment is carried outagain after the step of fabricating the crystalline silicon filmaccording to embodiment 1 shown in FIG. 1. When heat treatment iscarried out at step shown in FIG. 1(C), nickel (metal element) in thecrystalline silicon film 105 is gradually absorbed into the amorphoussilicon film 107 as shown in FIG. 4. At this time, the density of nickelin the vicinity of the surface of the crystalline silicon film 105becomes higher than the nickel density in the vicinity of the interfaceof the silicon oxide film 102 on the lower surface of the crystallinesilicon film 105. This means that nickel in the crystalline silicon film105 is sucked out into the amorphous silicon film 107 with the resultthat nickel element is deviated to the surface side of the crystallinesilicon film 105.

Consequently, in the case wherein a thin film transistor is fabricatedby using a crystalline silicon film 108 which is formed on a glass plate101 as shown in FIG. 1(D), carriers are to be conducted on the surfaceof the crystalline silicon film 108. It is not preferable that a highdensity of nickel exists in an area where carriers are conducted.

Then, in embodiment 6, after a state shown in FIG. 1(D) is obtained,heat treatment is conducted so that nickel is dispersed again in thecrystalline silicon film 108. In the heat treatment discussed here,nickel may be dispersed. Therefore, the temperature may be set to 400°C. or more. Further, the upper limit of the temperature is limited bythe heat resistance of the glass substrate 101. Consequently, theheating temperature used in embodiment 6 is 400° C. or more, and thetemperature may be less than the strain point of the glass substrate.

Details of embodiment 6 will be explained in detail by using FIG. 8. Inthe beginning, after passing through the fabrication step shown in FIG.1, a state shown in FIG. 1(D) will be obtained. The state in FIG. 1(D)is described in FIG. 8(A). FIG. 8(A) shows a layer 802 (on the surfaceside) in which nickel is segregated and a high density of nickel iscontained, and a layer 801 in which nickel is contained with a densitylower than that of the layer side denoted by reference numeral 802.These layers 801 and 802 constitute a crystalline silicon film 108 (seeFIG. 1(D)) formed on the glass substrate 101 via a base film 102.

In a state shown in FIG. 8(A), the crystalline silicon film is heattreated. Here the film is subjected to two hour heat treatment at 500°C. Consequently, the nickel element in an area denoted by referencenumeral 802 is dispersed into an area denoted by reference numeral 801where the nickel element is present at a low density. In this manner,this area 802 can be in a state in which no segregation of nickel ispresent. Then, the crystalline silicon film 803 can be obtained wherethe nickel density on the surface can be lowered. (FIG. 8(C))

Embodiment 7

Embodiment 7 is an example in which laser light is irradiated in placeof heat treatment at the step of crystallization in the structure shownin embodiment 1. The step of fabricating embodiment 7 is shown in FIG.9. In the case of Embodiment 7, a layer denoted by reference number 802comprising a crystalline silicon film where nickel is present at a highdensity in the surface of the crystalline silicon film, and a layer 801comprising a crystalline silicon film in which the nickel element ispresent at a low density are obtained. (FIG. 9(A))

Next, a laser light is irradiated to disperse the nickel element fromthe layer 802 to the layer 801. (FIG. 9(B)) Then, a crystalline siliconfilm 901 is obtained which has a state in which nickel is uniformlydispersed in the film. (FIG. 9(C))

Embodiment 8

Embodiment 8 is characterized in that an amorphous silicon film 107shown in FIG. 1(C) is formed in a state in which the defect density isartificially high at the step shown in embodiment 1. At step shown inembodiment 1, the crystalline silicon film 105 contains an average ofabout 3×10¹⁸ atom cm⁻³ of the nickel element as shown in FIG. 2. Then,embodiment 7 is characterized by heightening the removing capabilitiesof the nickel element by setting the defect density in the amorphoussilicon film 107 at least to a level more than the aforementioneddensity of the nickel element.

The defect density in the amorphous silicon film 107 can be estimated bymeasuring the spin density. In addition, to artificially form defects,the sputtering method, plasma CVD at low temperature may be used.Otherwise the plasma CVD or low pressure thermal CVD using only silaneand disilane may be used without using hydrogen for neutralizingunpaired bonds.

When the defect density of the amorphous silicon film 17 is set to ahigh level, the removing capabilities of nickel element can be furtherenlarged. Then the effect shown in FIGS. 3 and 4 can be made large.

Embodiment 9

In this embodiment, a polycrystalline silicon film is used as a nickelelement diffusion film for absorbing nickel element thereinto.

A process for fabricating a crystalline silicon film in accordance withthis embodiment is described with reference to FIG. 1. First, a siliconoxide film 102 is formed as a base film to a thickness of 3000 Å onCorning 7059 glass substrate 101 (having a strain point 593° C.).

Next, an amorphous silicon film 103 is formed to a thickness of 600 Å byplasma CVD or low pressure thermal CVD. Nickel acetate solution adjustedto a prescribed nickel concentration is dripped on the amorphous siliconfilm 103 and is spin-coated by spinner 100 to form an aqueous film 104.In this way, nickel is provided in contact with a surface of theamorphous silicon film 103. (FIG. 1(A))

Next, the amorphous silicon film 103 is crystallized by heat treatmentto obtain a crystalline silicon film 105. Temperature of the heattreatment is 550° C. and the duration of the heat treatment is 4 hours.(FIG. 1(B))

A silicon oxide film 106 is formed to a thickness of several tens Å to100 Å on a surface of the obtained crystalline silicon film 105 by a UVlight irradiation in air

Next, a polycrystalline silicon film 107 is formed to a thickness of 600Å by low pressure thermal CVD. It is not necessary to form thepolycrystalline silicon film 107 with a film quality required for anactive layer of a semiconductor. The actual film has a high defectdensity. It is preferred that this defect density is higher than defectdensity of the crystalline silicon film 105.

Next, nickel element contained in the crystalline silicon film 105 isdiffused into the polycrystalline silicon film 107 through the oxidefilm 106 by heat treatment. (FIG. 1(C)).

The lower limit of the heating temperature of this heat treatment isdefined as a temperature at which nickel can diffuse and the lower limitis 400° C. or higher. The upper limit is defined as a strain point ofthe glass substrate 101. Nickel element contained in the crystallinesilicon film 105 diffuses into the polycrystalline silicon film 107 bythis heat treatment to enable the nickel element concentration in thecrystalline silicon film 105 to decrease.

Generally, by forming the polycrystalline silicon film 107 to athickness more than that of the crystalline silicon film 105, nickelconcentration in the crystalline silicon film 105 can be lowered to halfor less by the heat treatment.

Then, the amorphous silicon film 107 is removed by etching. Hydradine(N₂H₆) or ClF₃ gas can be used. Since the etching rate of the siliconoxide is extremely low and the silicon oxide film 106 serves as anetching stopper, only the polycrystalline silicon film 107 which hasabsorbed nickel can selectively be removed.

Next, the silicon oxide film 106 is removed by buffered hydrofluoricacid and fluorine nitrate to obtain a crystalline silicon film 108 in alow nickel element concentration therein, as shown in FIG. 1(D).

Embodiment 10

An amorphous Si_(x)Fe_(1-x) film (0<x<1) is used as a nickel elementdiffusion film for absorbing nickel element therein in this embodiment.

A process for fabricating a crystalline silicon film according to thisembodiment is described with reference to FIG. 1. First, a silicon oxidefilm 102 is formed as a base film to a thickness of 3000 Å on Corning7059 glass substrate 101 having a strain point of 593° C.

Next, an amorphous silicon film 103 is formed to a thickness of 600 Å byplasma CVD or low pressure thermal CVD. A nickel acetate solutionadjusted to a prescribed nickel concentration is dripped on theamorphous silicon film 103, and spin-coated by spinner 100 to form anaqueous film 104. In this way, nickel element is provided in contactwith a surface of the amorphous silicon film. (FIG. 1(A))

Next, the amorphous silicon film 103 is crystallized by heat treatmentto obtain a crystalline silicon film 105. The heating temperature is550° C. and the heating duration is 4 hours. (FIG. 1(B)).

A silicon oxide film 106 is formed to a thickness of several tens A to100 Å on a surface of the obtained crystalline silicon film 105 by a UVlight irradiation in air.

Next, an amorphous Si_(x)Ge_(1-x) film 107 is formed to a thickness of600 Å by plasma CVD using a silane (SiH₄) and germane (GeH₄) as a rawmaterial gas. In order to obtain the amorphous Si_(x)Ge_(1-x) film 107at a high defect density, low temperature is used as the substratetemperature during the film formation and the raw material gas is usedwithout being diluted with hydrogen.

Next, nickel element contained in the crystalline silicon film 105 isdiffused into the amorphous Si_(x)Ge_(1-x) film 107 through the oxidefilm 106 by heat treatment. (FIG. 1(C)).

The lower limit of the heating temperature of this heat treatment isdefined as a temperature at which nickel can be diffused. The lowerlimit is 400° C. or higher. The upper limit is defined as a strain pointof the glass substrate 101. The nickel element contained in thecrystalline silicon film 105 is diffused into the amorphousSi_(x)Ge_(1-x) film 107 to lower nickel element concentration in thecrystalline silicon film 105.

The Si_(x)Ge_(1-x) film 107 is removed by etching. In order to use thesilicon oxide film 106 as an etching stopper, an etching solution oretching gas having a high etching selectivity between the Si_(x)Ge_(1-x)film 107 and the silicon oxide film 106 is used. In this way, only theSi_(x)Ge_(1-x) film 107 which has absorbed nickel can be selectivelyremoved.

Next, the silicon oxide film 106 is removed by buffered hydrofluoricacid or fluorine nitrate to obtain a crystalline silicon film 108 with alow nickel element concentration, as shown in FIG. 1(D).

As described above, in accordance with the present invention, by thefunction of the metal element, the crystalline silicon film can befabricated at a low temperature such as about 550° C. or less which islower than the prior art. Consequently, the crystalline silicon filmusing a glass substrate can be obtained.

Further, a crystalline silicon film having a low density of metalelement can be obtained by dispersing the metal element into theamorphous silicon film from the crystalline silicon film crystallizedwith the action of the metal element. Consequently, a device free fromthe bad influence of the metal element, for example, a thin filmtransistor can be obtained by using a crystalline silicon film.

Further, the crystalline silicon film free from the deviation of themetal element can be obtained by dispelling to a second silicon film aportion where the metal element in the silicon film crystallized by theaction of the metal element is deviated. In this manner, a device freefrom the bad influence of the metal element can be obtained.

What is claimed is:
 1. A method for fabricating a semiconductor thinfilm comprising the steps of: providing an amorphous silicon film with ametal element which promotes crystallization of said semiconductor film;crystallizing said amorphous silicon film by heat treatment to obtain acrystalline semiconductor film; forming a silicon nitride film incontact with said crystalline semiconductor film; forming a metalelement diffusion film comprising a semiconductor in contact with saidsilicon nitride film; diffusing said metal element into said metalelement diffusion film; and removing said metal element diffusion filminto which said metal element has been diffused, using said siliconnitride film as an etching stopper.
 2. A method for fabricating asemiconductor thin film comprising the steps of: providing an amorphoussilicon film with a metal element which promotes crystallization of saidsilicon film; obtaining a crystalline silicon film by crystallizing saidamorphous silicon film by heat treatment; forming a silicon nitride filmin contact with said crystalline silicon film; forming an amorphoussemiconductor film in contact with said silicon nitride film; dispersingby heat treatment the metal element in said crystalline silicon filminto the smorphous semiconductor film formed in contact with saidsilicon nitride film; and removing the semiconductor film formed incontact with said silicon nitride film by using said silicon nitridefilm as an etching stopper.
 3. The method of claim 2 wherein saidamorphous semiconductor film comprises silicon.
 4. A method forfabricating a semiconductor thin film comprising the steps of: providingan amorphous silicon film with a metal element which promotescrystallization of said silicon film; obtaining a crystalline siliconfilm by crystallizing said amorphous silicon film by heat treatment;forming a silicon nitride film in contact with said crystalline siliconfilm; forming an amorphous semiconductor film in contact with saidsilicon nitride film; crystallizing by heat treatment the amorphoussemiconductor film which is formed in contact with said silicon nitridefilm; and removing the semiconductor film which is formed in contactwith said silicon nitride film by using said silicon nitride film as anetching stopper.
 5. The method of claim 4 wherein said amorphoussemiconductor film comprises silicon.
 6. A method for fabricating asemiconductor thin film comprising the steps of: providing an amorphoussemiconductor film with a metal element which promotes crystallizationof said semiconductor film; obtaining said amorphous semiconductor filmby heat treatment to obtain a crystalline semiconductor film; forming asilicon nitride film in contact with said semiconductor film; forming ametal element diffusion film comprising a semiconductor in contact withsaid silicon nitride film; diffusing said metal element into said metalelement diffusing film; and removing said metal element diffusion filminto which said metal element has been diffused.
 7. A method forfabricating a semiconductor device comprising the steps of: providing anamorphous semiconductor film with a metal element which promotescrystallization of said semiconductor film; crystallizing said amorphoussemiconductor film to obtain a crystalline semiconductor film; formingan etching stopper film on said crystalline semiconductor film; forminga metal element diffusion film comprising a semiconductor on saidetching stopper film; diffusing said metal element into said metalelement diffusion film from said crystalline semiconductor film; andremoving said metal element diffusion film after diffusing said metalelement thereto.
 8. The method according to claim 7 wherein said metalelement is selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd,Os, Ir, Pt, Cu and Au.
 9. A method for fabricating a semiconductordevice comprising the steps of: providing an amorphous semiconductorfilm with a metal element which promotes crystallization of saidsemiconductor film; crystallizing said amorphous semiconductor film toobtain a crystalline semiconductor film; forming an etching stopper filmon said crystalline semiconductor film; forming a metal elementdiffusion film comprising silicon on said etching stopper film;diffusing said metal element into said metal element diffusion film fromsaid crystalline semiconductor film; and removing said metal elementdiffusion film after diffusing said metal element.
 10. The methodaccording to claim 9 wherein said metal element diffusion film comprisesamorphous silicon.
 11. The method according to claim 9 wherein saidmetal element diffusion film comprises polycrystalline silicon.
 12. Themethod according to claim 9 wherein said metal element diffusion filmcomprises amorphous Si_(x) Ge _(1-x) ( 0<x<1 ).
 13. The method accordingto claim 9 wherein said metal element is selected from the groupconsisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.
 14. Amethod for fabricating a semiconductor device comprising the steps of:providing an amorphous semiconductor film with a metal element whichpromotes crystallization of said semiconductor film; crystallizing saidamorphous semiconductor film to obtain a crystalline semiconductor film;forming an etching stopper film on said crystalline semiconductor film;forming a gettering film comprising silicon by sputtering on saidetching stopper film; diffusing said metal element into said getteringfilm from said crystalline semiconductor film; and removing saidgettering film after diffusing said metal element.
 15. The methodaccording to claim 14 wherein said etching stopper film comprisessilicon oxide.
 16. The method according to claim 14 wherein said etchingstopper film comprises silicon nitride.
 17. The method according toclaim 14 wherein said gettering film comprises amorphous silicon. 18.The method according to claim 14 wherein said gettering film comprisespolycrystalline silicon.
 19. The method according to claim 14 whereinsaid gettering film comprises amorphous Si_(x) Ge _(1-x) ( 0<x<1 ). 20.The method according to claim 14 wherein said metal element is selectedfrom the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu andAu.
 21. A method for fabricating a semiconductor device comprising thesteps of: providing an amorphous semiconductor film with a metal elementwhich promotes crystallization of said semiconductor film; crystallizingsaid amorphous semiconductor film to obtain a crystalline semiconductorfilm; forming an etching stopper film on said crystalline semiconductorfilm; forming a gettering film comprising silicon on said etchingstopper film; diffusing said metal element into said gettering film fromsaid crystalline semiconductor film; and removing said gettering filmafter diffusing said metal element, wherein said gettering film is soformed that a defect density in the gettering film is higher than adensity of said metal element in the crystalline semiconductor film. 22.The method according to claim 21 wherein said etching stopper filmcomprises silicon oxide.
 23. The method according to claim 21 whereinsaid etching stopper film comprises silicon nitride.
 24. The methodaccording to claim 21 wherein said gettering film comprises amorphoussilicon.
 25. The method according to claim 21 wherein said getteringfilm comprises polycrystalline silicon.
 26. The method according toclaim 21 wherein said gettering film comprises amorphous Si_(x) Ge_(1-x) ( 0<x<1 ).
 27. The method according to claim 21 wherein saidmetal element is selected from the group consisting of Fe, Co, Ni, Ru,Rh, Pd, Os, Ir, Pt, Cu and Au.
 28. A method for fabricating asemiconductor device comprising the steps of: providing an amorphoussemiconductor film with a metal element which promotes crystallizationof said semiconductor film; crystallizing said amorphous semiconductorfilm to obtain a crystalline semiconductor film; forming an oxide filmon the crystalline semiconductor film by oxidizing a surface of saidcrystalline semiconductor film; forming a gettering film comprisingsilicon on said oxide film; diffusing said metal element into saidgettering film from said crystalline semiconductor film; and removingsaid gettering film after diffusing said metal element.
 29. The methodaccording to claim 28 wherein said gettering film comprises amorphoussilicon.
 30. The method according to claim 28 wherein said getteringfilm comprises polycrystalline silicon.
 31. The method according toclaim 28 wherein said gettering film comprises amorphous Si_(x) Ge_(1-x) ( 0<x<1 ).
 32. The method according to claim 28 wherein saidmetal element is selected from the group consisting of Fe, Co, Ni, Ru,Rh, Pd, Os, Ir, Pt, Cu and Au.
 33. A method for fabricating asemiconductor device comprising the steps of: providing an amorphoussemiconductor film with a metal element which promotes crystallizationof said semiconductor film; crystallizing said amorphous semiconductorfilm to obtain a crystalline semiconductor film; forming an oxide filmon the crystalline semiconductor film by oxidizing a surface of saidcrystalline semiconductor film; forming a gettering film comprisingsilicon by sputtering on said oxide film; and diffusing said metalelement into said gettering film from said crystalline semiconductorfilm; and removing said gettering film after diffusing said metalelement.
 34. The method according to claim 33 wherein said getteringfilm comprises amorphous silicon.
 35. The method according to claim 33wherein said gettering film comprises polycrystalline silicon.
 36. Themethod according to claim 33 wherein said gettering film comprisesamorphous Si_(x) Ge _(1-x) ( 0<x<1 ).
 37. The method according to claim33 wherein said metal element is selected from the group consisting ofFe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.
 38. A method forfabricating a semiconductor device comprising the steps of: providing anamorphous semiconductor film with a metal element which promotescrystallization of said semiconductor film; crystallizing said amorphoussemiconductor film to obtain a crystalline semiconductor film; formingan oxide film on the crystalline semiconductor film by oxidizing asurface of said crystalline semiconductor film; forming a gettering filmcomprising silicon on said oxide film; diffusing said metal element intosaid gettering film from said crystalline semiconductor film; andremoving said gettering film after diffusing said metal element, whereinsaid gettering film is so formed that a defect density in the getteringfilm is higher than a density of said metal element in the crystallinesemiconductor film.
 39. The method according to claim 38 wherein saidgettering film comprises amorphous silicon.
 40. The method according toclaim 38 wherein said gettering film comprises polycrystalline silicon.41. The method according to claim 38 wherein said gettering filmcomprises amorphous Si_(x) Ge _(1-x) ( 0<x<1 ).
 42. The method accordingto claim 38 wherein said metal element is selected from the groupconsisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.